The present invention relates to a heat-resistant springmade of a fiber-reinforced metal (hereinafter, referred to as "FRM"). More particularly, it relates to a novel light spring made of a fiber-reinforced metallic composite material, which has an excellent heat-resistance, i.e. dynamic and mechanical properties such as strength and modulus at elongation, bending or compression, the fatigue strength of the spring material not being deteriorated even at a lower or higher temperature as well as at room temperature.
A spring is usually used as a part or element of various machines and apparatus and is produced by forming an elastic material which is capable of absorbing an external force as an elastic energy into an appropriate shape. The spring is roughly classified into (1) a metallic spring (e.g. steel spring, non-ferrous metallic spring, etc.) and (2) a non-metallic spring (e.g. rubber spring, fluidic spring, etc.)
In order to select the most suitable spring for individual utility, various factors such as elastic coefficient, elastic limit, fatigue strength, heat resistance, corrosion resistance, fabrication quality, or coefficient of thermal expansion of the material should be taken into consideration.
As the metallic spring, there are known steel springs such as spring made of carbon steel or alloyed steel. These steel springs are suitable for practical use in view of excellent fabrication quality and are used in various shapes. However, these steel springs have drawbacks in that they have too large of a density such as 8-9 g/cm.sup.3 and hence the strength and elasticity thereof are significantly decreased at a high temperature. For example, a carbon steel spring shows a working limit of temperature at 180.degree. C., and even a stainless steel spring shows the working limit at about 310.degree. C. (cf. "Spring Design" 2nd Ed., edited by Spring Technique Research Committee in Japan, issued by Maruzen, 1963, page 9). In order to improve the heat resistance of springs there may be used some non-ferrous metallic springs such as a copper alloyed spring, cobalt- or nickel-base spring. However, these non-ferrous metallic springs show still decreased strength and modulus of elasticity at a higher temperature. Thus, there has never been obtained a light, heat-resistant spring material having a large specific strength (i.e. strength/density) and a large specific modulus of elasticity (i.e. modulus of elasticity/density). Moreover, the conventional metallic springs usually rapidly decrease a fatigue breaking stress thereof at a repeating time of 10.sup.6 times in fatigue test and hence they are hardly practically useful. Besides, a carbon steel spring is also inferior in corrosion resistance, and a steel spring shows unfavorably decreased absorption energy at a comparatively low temperature such as lower than the transition temperature of steel because of the rapid breaking thereof, that is, they show so-called brittleness at low temperature. (cf. "Springs" 2nd Ed., edited by Spring Technique Research Committee in Japan, issued by Maruzen, 1970, page 278).
As the non-metallic spring, there are known the rubber spring, fluidic spring, and further, fiber-reinforced resin (FRP) spring. (cf. Japanese Patent Publication (unexamined) Nos. 33962/1977, 33963/1977, 34161/1977, 36250/1977, and 56252/1977). As a spring made of FRP, there are known, for example, springs made of a thermosetting or thermoplastic resin which is reinforced with glass fiber or carbon fiber. The FRP is comparatively easily fabricated and has light weight and excellent mechanical properties, but it has a most important problem, that is it has a low heat resistance. For example, the spring made of polyimide resin reinforced by glass fiber has a heat resistance of lower than 300.degree. C.
It has recently been feared that energy and resources in the world will be exhausted in the near future, and hence, it is necessary to save energy and resources in various fields. From this viewpoint, it is required to reduce the weight of various parts or elements of various transportations such as automobiles, airplanes, railway cars, and further various other machines and apparatuses. Moreover, from the standpoint of saving energy by enhancing the energy effeciency and giving high performance to the machines and apparatuses, it is required to improve the heat resistance of the parts and elements. By weight-saving or improving the heat resistance of the parts or elements, not only the parts or elements themselves but also whole machines or apparatuses or whole systems to which the parts or elements are incorporated will be improved. Accordingly, if there can be obtained a spring having an excellent heat resistance (i.e. the spring having similar mechanical properties even at a lower or higher temperature as those at room temperature) and having also light weight, it will be revolutionary in the machine and relative industries.